Document the reasons for using Mock / key features

[praiskup]

We frequently explain why plain "rpmbuild call in podman/buildah" isn't a replacement for Mock. It would be nice to have documentation documenting the key benefits of Mock from this perspective (with some technical details).

/note for myself: ADR

[xsuchy]

Notes for later:

• Mock prepares directory structure for rpmbuild https://github.com/rpm-software-management/mock/blob/main/mock/py/mockbuild/buildroot.py#L783
• Mock prepares fuse device https://github.com/rpm-software-management/mock/blob/main/mock/py/mockbuild/buildroot.py#L810 (important for rpm-ostree builds)
• Mock setup structure for build-specific macros https://github.com/rpm-software-management/mock/blob/main/mock/py/mockbuild/buildroot.py#L763 and other places
• Mock prepares special devices that are needed for various packages to succeed the build https://github.com/rpm-software-management/mock/blob/main/mock/py/mockbuild/buildroot.py#L833
• Mock correctly setup time settings https://github.com/rpm-software-management/mock/blob/main/mock/py/mockbuild/buildroot.py#L509